MESFETs (metal semiconductor field effect transistors) include a conducting channel positioned between source and drain regions. Carrier flow from the source to drain is controlled by a Schottky metal gate. The channel is controlled by varying the depletion layer width below the metal contact which modulates the thickness of the conducting channel and thereby the current. Current power transistors based on GaN are constructed mostly as HEMTs (high electron mobility transistors) which are also known as heterostructure FETs (HFETs) or modulation-doped FETs (MODFETs). An HEMT is a field effect transistor with a junction between two materials having different band gaps such as GaN and AlGaN which forms the channel instead of a doped region such as in a MOSFET (metal oxide semiconductor field effect transistor). HEMTs provide a two-dimensional electron gas (2DEG) which is formed on the boundary between e.g. an AlGaN barrier layer and a GaN buffer layer. Without further measures, such a construction leads to a self-conducting i.e. normally on transistor. That is, the HEMT conducts in the absence of a positive gate voltage.
Conventional normally-on GaN HEMTs typically make use of a top field plate connected to the source terminal in order to lower the electric field peaks within the device, which in turn increases the breakdown voltage of the device. The top metal field plate is disposed above the gate electrode and insulated from the gate electrode by a dielectric material. The top metal field plate not only affects the electric field distribution in a GaN HEMT device, but also deeply impacts the AC behaviour of the device. Indeed, the main capacitance of the transistor can be modified and the switching performance of the transistor affected accordingly. The top metal field plate can also alleviate current ‘collapse’ which typically arises due to high concentrations of traps/defects present in GaN-based devices that induce large variation in the current drive capability of the transistor during switching cycles, by lowering the horizontal and vertical electric fields and reducing, as a consequence, the field-related trapping and de-trapping mechanisms. It is desirable to have a more efficient field plate which increases the breakdown strength of a GaN HEMT by shaping the electric field in such a way to lower the maximum electric field peaks and to enhance the breakdown strength of the device.